This invention relates to switchable optical components, particularly ones utilizing guided wave optics and to ones particularly adapted for use in wavelength division mutliplexing (WDM) systems, including switchable waveguide gratings, particularly ones used for switchable add/drop filtering (SADF) and wavelength selective cross-connect (WSXC), to designs utilizing coupler-halves or side-polished fibers, to attenuators and other optical grating components utilizing very small period optical grating elements, to other wavelength selective or wavelength independent components, to devices using such components, to methods for the fabrication of such components and devices, and to integrated structures for such devices.
Because of their very large bandwidth capacity, optical signals are being increasingly utilized for the transmission of data. Further, the bandwidth capacity of a given fiber optic cable can be further increased by transporting a multiplicity of independent signals within a single fiber on separate channels at slightly different wavelengths, a technique known as wavelength division multiplexing (WDM). Thus, for example, a nominally 1550 nm fiber optics signal might comprise four, eight, 64, 80 or more channels, each separated by for example approximately 0.8 nm (corresponding to 100 GHz) or approximately 1.6 nm (corresponding to 200 GHz). Multiwavelength operation facilitates an increasingly important advantage of optical transport and switching which is that several non-interacting signals may pass through the switch simultaneously, which signals convey entirely incompatible data rates, encodings and protocols in parallel without compromising one another. However, for such signals to be useful, it must be possible to wavelength selectively switch the optical signals coming in on an optical fiber, bus (or other optical conduit) to a fiber/conduit leading to a desired drop/destination, to wavelength selectively add signals from a drop to the bus or to wavelength selectively transfer signals between fibers or other optical conduits. The first two functions are sometimes referred to as switchable add/drop filtering (SADF) and the last function is sometimes called wavelength selective cross-connect (WSXC). In other applications, switching the entire fiber signal, inclusive of all wavelength channels, is required (such switching sometimes being denoted as xe2x80x9cspace switchingxe2x80x9d). In complex fiber optic structures such as those used in the telecommunications industry and for sensor and computer data networks, light signals must be efficiently routed or switched from an array of N incoming optical fibers, which fibers may be single mode or multimode, to an array of M outgoing optical fibers. Such a space switch will sometime be referred to hereinafter as an Nxc3x97M switch or cross-connect.
While a number of techniques have been proposed over the years for performing Nxc3x97M switching optically, none of these techniques have proved to meet all requirements simultaneously. This is partly due to the varied architectures which are required for such switches. For example, an N fiber in, N Fiber out (Nxc3x97N) switch that maps each incoming fiber optical signal to one and only one fiber output is termed an Nxc3x97N cross-connect. It is nonblocking if any connection is possible, without regard to earlier established connections. For some applications, reconfigurably nonblocking switches are sufficient. In other applications, switches that multicast or broadcast, sending one incoming signal to more than one output, or that perform other variant functions, are required. The data capacity demands on fiber optic networks are also becoming more complex, imposing a requirement that switching technologies be scalable so as to be extendable in a straight forward manner from small switches (for example 2xc3x972 or 4xc3x974 to larger switches such as 64xc3x9764, 1024xc3x971024, and beyond). It is also desirable that such switches be integrable such that individual miniaturized switching elements can be combined with many others on a single chip or substrate to provide a larger Nxc3x97N or Nxc3x97M cross-connect structure. However, designing such structures, particularly for larger switches, is very complex even for single channel operation, and the complexity increases dramatically for multichannel WDM operation (i.e., wavelength selective switching with an Nxc3x97Mxc3x97m switch, where m is the number of WDM channels).
Another requirement for optical switches of the type described above in particular, and for optical components and structures in general, is that they efficiently interface with optical fibers, the use of which to transport high bandwidth signals over long distances is increasingly prevalent, in a manner so as to minimize coupling losses. Other key performance parameters include minimizing insertion loss, crosstalk and polarization sensitivity, insuring good optical isolation in all switch states, good spectral bandwidth, and good dynamic range for on/off contrast ratio. Low operating power, high switching speed, low power consumption, stability, long service life/temperature insensitivity and high reliability are also important. However, for many network reconfiguration and protection switching functions, switching speeds in the range of 1 microsecond to 1 millisecond are adequate and sufficient.
Further, in the present state of the art, neither space switching, nor wavelength selective switching techniques, are entirely satisfactory. One reason for this is that the various network control and reconfiguration functions required have generally been met by different and incompatible technologies. Optical network systems would be considerably advanced, in efficiency, manufacturability and cost, if several disparate network control functions could be implemented on the basis of a single underlying technology.
All-optical switching is increasingly regarded as essential for future networks. Because satisfactory products for performing such optical switching have not existed, it has therefor been necessary to convert optical signals to be switched into electrical signals for switching and to then reconvert the signals to optical signals for outputting. This technique can be expensive, time consuming, impose bandwidth limitations on the system and introduce several sources of potential error. It can also limit the flexibility of the system and is generally not an efficient way to operate.
In addition to the switching applications discussed above, there are numerous applications where a need exists to be able to change the direction in which an optical signal is passing through a waveguide, dynamically filter an optical signal, particularly a multiwavelength or multichannel signal, so as to selectively add, drop, pass or block various of the individual wavelengths or channels (or the entire signal), to selectively attenuate an optical signal, including one or more signals of a multiwavelength or multichannel line, to selectively crossconnect optical paths including multichannel or multiwavelength optical paths to facilitate the transfer of one or more channels therebetween, and/or to selectively couple the multiwavelengths or multichannels along optical paths out of the plane of the waveguide. It should be possible to perform all of these functions utilizing optical components and/or structures which are relatively easy and inexpensive to fabricate. In particular, it would be desirable if fabrication techniques could be provided which would permit complex optical networks to be fabricated utilizing a parallel, simultaneous, one-shot fabrication techniques that incorporates a multiplicity of functionalities on a single chip for the implementation of space switching, wavelength selective switching, switchable add-drop filtering, wavelength selective cross-connect switching, together with such additional functions as programmable attenuation, all on a single chip and using a single material technology rather than requiring each component to be separately fabricated.
In the following sections, various terms will be used, which terms should be considered to have the following definitions:
xe2x80x9cBragg gratings or gratingsxe2x80x9d are periodic structures formed by spatially varying refractive index distributions or similar perturbations throughout a defined volume or the boundary of a guiding region. Simple Bragg gratings are periodic in one dimension. More complex diffractive structures, which for purposes of this invention will also be encompassed within this definition, may be volume holograms, diffractive lenses, or other computer generated or optically recorded diffractive index distributions, in most cases permeating a substantially three-dimensional volume, designed and fabricated for purposes of coupling an incident laser or other light beam or a light beam received through guided wave optics into a desired output state or mode either one guided mode to another guided mode, a guided mode to a free space mode or vice versa.
xe2x80x9cSwitchable gratings or switchable Bragg gratingsxe2x80x9d are volumetric gratings whose grating period index variation can be modulated, induced or caused to vanish by application of an electric field. These differ from standard gratings which are not switchable. This definition does not imply altering the period of the grating, but only the amplitude of the spatially varying index variation. A switchable grating, in the simplest example, may be described as a grating element, that, in its switched-on state, has filtering or other diffractive properties comparable to a high quality conventional fiber or waveguide Bragg grating, of either the transmission or reflection type, and in its switched-off state, effectively vanishes to be replaced with a low loss waveguide or volume of transparent optical material.
Until recently, few mechanism were available for switchable Bragg gratings. Certain semiconductor gratings can be switched, but only in limited geometrical configurations, and the dynamic range for control of the spatial index modulation is relatively small. Liquid crystal gratings, usually formed by physically structured electrodes, may be switchable, but are primarily relevant to free space non-volumetric gratings, are excessively scattering for use with fiber optic signals and are relatively slow, switching being in the millisecond range. All manner of switchable gratings that involve the use of structured electrodes to produce the spatial periodicity, such as magneto-optic materials and lithium niobate materials, are limited in their application in that the spatial period and depth of grating are dependent on the lithographic processes of fabricating electrode patterns. Such structured electrode gratings are not practical at spatial periodicities much less than one micrometer, which smaller periods are essential in various fiber optic applications, and also do not tend to produce volumetric (Bragg) gratings since the electrode periods cannot penetrate unlimited volumes.
xe2x80x9cHolographic polymer/dispersed liquid crystals or H-PDLCxe2x80x9d are any microdroplet composite of liquid crystal (LC) in polymers or other morphological variants including polymer networks with interpenetrating LC. This family of switchable gratings is based on microdroplet dispersions of liquid crystals in a polymer host, the volume gratings containing periodic structures with periods as small as 200 nanometers or less which are achieved by holographic recording and photopolymerization processes. The switching of such gratings is achieved by applying an electrical field by means of a uniform, monolithic electrode to the entire grating region, as opposed to producing the grating by patterning the electrodes. Examples of H-PDLC include, but are in no way restricted to, the following:
1. R. L. Sutherland, L. V. Natarajan, V. P. Tondiglia, T. J. Bunning, Bragg Gratings in an Acrylate Polymer Consisting of Periodic Polymer-Dispersed Liquid-Crystal Planes, Chem. of Materials, 1993, 5, 1533.38.
2. R. L. Sutherland, L. V. Natarajan, V. P. Tondiglia, T. J. Bunning and W. W. Adams, Development of Photopolymer/Liquid Crystal composite Materials for Dynamic Hologram Applications, Proc. SPIE Vol. 2152, paper 38.
3. V. P. Tondiglia, L. V. Natarajan, R. L. Sutherland, T. J. Bunning and W. W. Adams, Volumn holographic image storage and electro-optic readout in a polymer dispersed liquid crystal film, Opt. Lett. v. 20, p. 1325, 1995.
4. R. L. Sutherland, L. V. Natarajan, V. P. Tondiglia, T. J. Bunning and W. W. Adams, Switchable holograms in a new photopolymer-liquid crystal composite, Proc. SPIE, Vol. 2404, p. 132, 1995.
5. R. L. Sutherland, L. V. Natarajan, V. P. Tondiglia, T. J. Bunning and W. W. Adams, Electrically switchable volumn gratings in PDLC, Appl. Phys. Lett., Vol. 64, p. 1074, 1994.
6. U.S. Pat. No. 4,938,568. Jul. 3, 1990. John D. Margerum, et al.
7. U.S. Pat. No. 5,096,282. Mar. 17, 1992. John D. Margerum, et al.
8. A. Golemme, B. L. Volodin, B. Kippelen, and N. Peyghambarian, Photorefractive Polymer-Dispersed Liquid Crystals, Optics Letters, Vol.22, No. 16, p. 1226-1228, Aug. 15, 1997.
9. Keiji Tanaka, Kinya Kato, Shinjui Tsuru, and Shigenobu Sakai, Holographically Formed Liquid-Crystal/polymer Device for Reflective Color Display, Journal of the SID, Vol 2, No. 1, p. 37-40, 1994.
10. Emily W. Nelson, Adrian D. Williams, Gregory P. Crawford, Louis D. Silverstein, and Thomas G. Fiske, Full-Color Reflective Displays, ISandT""s 50th Annual Conference, p. 669-673.
11. G. Crawford and S. Zumer, Liquid Crystals in Complex Geometries Formed by Polymer and Porous Networks, Taylor and Francis, 1996, London.
xe2x80x9cElectronically switchable Bragg grating or (ESBG)xe2x80x9d any of an extensive range of devices and device geometries realized utilizing the H-PDLC materials technology.
In accordance with the above, this invention provides a component for selectively reconfiguring an optical signal on a guided wave optical path which path may for example be a waveguide formed in a composite optical structure. The component includes at least one ESBG in optical contact with the path, and electrodes for selectively applying at least a first and a second voltage across the ESBG, there being no change in the optical signal for the path when the first voltages is across the ESBG and there being a selected change in the optical signal on the path when the second voltage is across the ESBG. Where the optical signal is a multichannel WDM signal, a selected channel may be dropped from the path when the second voltage is across the ESBG, the dropping occurring either by the channel being reflected back through the path or being dropped/outcoupled from the path through the ESBG.
The component may include a second guided wave optical path which is optically coupled to the ESBG, with at least one selected channel being transferred between the paths when the second voltage is across the ESBG. In particular, a channel may be either dropped or added to the optical path by being transferred between the optical path and the second optical path through the ESBG when the ESBG has a second voltage thereacross. Where WDM signals appear on both paths, WSXC transfer of at least a selected channel may be performed between the paths when a second voltage is across the ESBG.
A plurality of ESBGs or ESBG components may be optically coupled to both paths, with a different channel being transferred between the paths by each ESBG when the second voltage is thereacross. For some embodiment of the invention, there is an ESBG in optical contact with each of the paths and at least one optical path interconnecting the ESBGs, there being two optical paths interconnecting the ESBGs to form a ring for a generally preferred embodiment. One of the rings may be provided between the paths for each WDM channel to be transferred between the paths. The two signal-carrying optical paths preferably have the same first effective index, with the optical paths of the rings having a second effective index different from that of the first index, the different effective indices being achieved for example by having the optical paths of the ring of different size than that of the main optical paths.
The optical paths may also have different indices which may be accomplished for example by having the optical paths be of different size. The difference in the indices of the gratings is preferably greater than or equal to the index contrast xcex94n of the ESBG grating.
Rather than being integrated optics, the component may also be a switchable coupler half device, for example a switchable drop filter, a switchable outcoupler, or a switchable attenuator. For such an attenuator, the ESBG has a submicron grating.
The ESBG may also be part of a resonator. When the resonator is being used in a channel drop filter, the component includes a first drop resonator and second reflector resonator spaced from each other in the direction of travel of the optical signal on the optical path by an integer number of wavelengths of a wavelength to be dropped, plus a half wavelength. Each resonator may also be a multipole resonator formed of resonator sections which are either series coupled or parallel coupled. The resonator component may also be a guide channel dropping filter with the resonator being between the optical paths. The resonator may also be a split resonator having a phase delay section between the split resonator sections.
Where there are two waveguides or optical paths, the ESBG may be in the cladding for both optical paths which claddings overlap. Where the claddings of the two optical paths do not overlap, the ESBG may extend over or overlie both optical paths to affect interconnection. For any component involving two optical paths, sidelobes may be suppressed by apodization. As previously indicated, all of the above are preferably effected through use of integrated optics technology except for the half coupler embodiments.
For half coupler embodiments, an optical fiber having a core with cladding therearound is provided, the core having an index n1, the cladding having an index n2, and the effective index of the fiber being ne, the cladding being at least partially removed in a selected region. An ESBG is mounted to the fiber in the region, the ESBG having an index nB when in a first state, where nB is substantially equal to n2. Electrical elements including electrodes are also provided for selectively applying a voltage across the ESBG, the effect which the ESBG has on light applied to the ESBG varying as the voltage thereacross changes, thereby changing the state of the ESBG. Light applied to the optical fiber is substantially unaffected by the ESBG when the ESBG is in its first state, this for example occurring when the mechanism applies substantially no voltage across the ESBG. Variation in nB as a result of voltage changes across the ESBG may result in selective attenuation of light applied to the fiber, particularly if the ESBG has a grating with a subwavelength. Such attenuation is generally substantially independent of wavelength.
The ESBG may cause light at one or more selective wavelengths to be coupled through the ESBG in at least one direction to or from the fiber as the voltage across the ESBG is varied from the voltage causing the ESBG to be in its first state. The ESBG grating may also have a period which causes light at one or more selected wavelengths to be reflected back along the fiber, thereby filtering such one or more selected wavelengths from light propagating in the fiber. Two of the fibers may be provided, with the regions of the fibers having cladding remove being adjacent and with an ESBG being mounted between the fibers in both regions. For this configuration, all light on one of the fibers passes unchanged through the ESBG to the other fiber when the ESBG has a selected index. When the fibers have substantially the same index, a light applied to one of the fibers is transferred through the ESBG to the other fiber when the ESBG is in its first state. When the ESBG is in a second state as a result of a voltage applied thereacross, light of at least one selected wavelength determined by the period of the grating is blocked from passing through the ESBG, such light continuing to propagate on the original fiber. The fibers may also have different indices in which case, for at least one wavelength of a multiwave light signal for which the condition 2xcfx80/xcex9=xcex2ixe2x88x92xcex2xe2x80x2i is satisfied, where xcex9 is the period of the grating and xcex2i, xcex2xe2x80x2i are the propagation constants for the two fibers respectively, there is coupling between the fibers only for such wavelength. Where the fibers have different indices, it is preferable that the difference in their effective index be greater than or equal to the index contrast xcex94n of the ESBG grating.
The invention also includes providing an ESBG having a subwavelength grating, which grating may have a period substantially less than 0.5 xcexcm. Such a subwavelength grating may be obtained by exposing a H-PDLC film by one of (a) exposing the film with two interfering light beams of suitable wavelength, the half angle xcex8 between the beams being large enough so that sin xcex8=xcex/2xcex9, where xcex is the center wavelength of a light signal with which the ESBG is to be utilized; (b) exposing the film through a suitable binary phase mask; (c) exposing the film through a master grating.
Integrated optical networks each having a plurality of nodes, with at least one ESBG formed at each node, may be formed by forming selected optical waveguides in a suitable substrate, at least selected ones of the waveguides passing through selected ones of the nodes, forming an ESBG with electrodes at each node and covering the waveguides and ESBGs. The ESBGs may be formed by forming an electrode film and a H-PDLC film at each node, and exposing the H-PDLC film at each node to form the ESBG grating thereon. The covering of the waveguides and ESBGs may be accomplished by forming a second electrode film on a cover plate at each node and covering the waveguides/H-PDLC film with the coverplate, each electrode on the coverplate overlying the H-PDLC film for the corresponding node. The exposing of the H-PDLC film may be accomplished by one of (a) exposing each H-PDLC film with two interfering light beams of suitable wavelengths, the beams being at a selected angle to each other; (b) exposing all the films simultaneously through a suitable binary phase mask; (c) and exposing each of the films through a suitable mask. Selected optical waveguides may also be formed in a first substrate and in a second substrate, which substrates are mounted adjacent each other during the covering step, the waveguides on the two substrates intersecting at at least selected nodes. For this embodiment, a H-PDLC film, with ESBG gratings formed therein at the nodes, is either formed on one of the substrates or independently formed and mounted between the substrates. An electrode film may be formed on each substrate on a waveguide at each node.
Finally, the invention includes an integrated optical network having N guided wave optical inputs and M guided wave optical outputs, the network including an optical waveguide connected to each input and to each output, the waveguides intersecting at nodes, and an ESBG switch component at each node which is operative to either pass an optical signal on one waveguide intersecting at the node on the waveguide or to transfer at least a portion of such optical signal to the other waveguide intersecting at the node, depending on the state of the ESBG. For WDM signals, an ESBG switch component may be provided at each node for each wavelength to be transferred at the node. For preferred embodiments, all the waveguides have substantially the same index n1, the ESBG switch component at each node includes an ESBG having a grating with an index xcex94n in optical contact with each waveguide and at least one waveguide interconnecting the ESBGS, the waveguide connecting the ESBGs having an index n2, where n1xe2x88x92n2xe2x89xa7xcex94n.
The foregoing and other objects, features and advantages of the inventions will be apparent from the following more particular description of preferred embodiments of the inventions as illustrated in the accompanying drawings.